1. Field of the Invention
The invention concerns a process for manufacturing a ductile, high strength, oxide dispersion hardened sintered alloy based on a metal with a high melting point, if necessary with small additions of substitution mixed-crystal phase which, however, do not have a serious effect on alloy properties, in which a metal oxide powder in dispersoid form is mixed with the basic metal powder, using oxides of those metals whose binding energy at temperatures &lt;0.5 T.sub.M is higher than that of the oxides of the basic metal.
2. Description of Related Art
Classical processes for altering the strength properties of metals include the forming of alloys via mixed-crystal phases and mechanical reshaping. In addition, it is known that the strength of materials produced by fusion metallurgy or powder metallurgy can be increased by introducing or removing dispersoids. According to the definition, dispersoids are particles, usually included in the metallic base matrix in a continuous fashion, which even at higher temperatures do not react with the basic metal or dissolve, and are not built into the base lattice as substitution metals. Particularly oxides, carbides and nitrides are used as dispersoids.
According to established doctrine, the disadvantage of dispersion hardening versus alloy hardening by continuous or discontinuous precipitation of a second phase within the basic phase from a common solution (precipitation hardening consists in the fact that "it is hardly possible to achieve the same degree of dispersion and strength increase as can be realized with precipitation processes in many cases" (H. Bohm, Introduction to Metallurgy, Hochschultaschenbucher Verlag, Mannheim, Zurich).
For producing dispersion-hardened alloys by processes of powder metallurgy, the dispersoids are usually introduced by soaking the powder with a dispersoid suspension, or by blending dispersoids in powder form with the basic metal powder.
Dispersoids introduced in this manner can be further homogenized by "mechanical alloying". The objective of mechanical alloying is to distribute the dispersoids as homogeneously as possible, even within the individual metal powder grains. These processes are very time-consuming and require grinding equipment of high quality. They are therefore very expensive, and their applicability depends on the state of the components. Moreover, practical application demands a compromise between the degree of homogenization and the cost of grinding, i.e. the grinding operation is limited in time.
The application DE-A1 35 4 255 contains a proposal for producing an ODS alloy by mixing the basic metal in the form of a salt solution with the dispersion particles in colloidal suspension and to finally reduce it to metal. As a special advantage, the finely distributed, homogeneous introduction of the dispersoid into the metal matrix is cited. However, even with this process, distribution is limited by the particle size of the components.
The production of dispersion hardened alloys consists in introducing particles as dispersoids which by definition do not react or alloy with the basic matrix. In connection with this fact, the sintered-metallurgy processes for producing dispersion alloys up until the present have used dispersoids with melting points that are usually considerably higher than the alloy sintering temperature. The dispersoids exist in the solid phase during the entire manufacturing process.
Due to the doctrine mentioned above, that dispersion hardening achieves only relatively small increases in strength, the additional means of mixed-crystal alloy hardening or precipitation hardening was applied in cases where greater mechanical strength was required. To achieve this, greater doses of additive metals were blended with the basic metals, next to dispersoids.
Next to powder metallurgy processes, it is known that oxide dispersion alloys of high-melting metals can be produced by fusion metallurgy, particularly by arc melting.
For instance, a process is known from DE-C1 12 90 727 for producing a niobium alloy of high strength by adding to the niobium small amounts of oxygen, carbon and/or nitrogen, plus possibly larger amounts of other high-melting metals, next to 0.5-12% zirconium. This alloy melted in the arc is then solution-annealed at 1600.degree.-2100.degree. for between 5 minutes and 9 hours, cooled, reshaped and finally subjected to precipitation annealing. The patent description states that, during solution annealing, the second phase--meaning the carbides, nitrides and/or oxides contained in the basic matrix (basic metal) after melting--forms a solution with the basic matrix. According to that invention, the second phase is to remain in solution during the cold shaping due to the solution annealing and subsequent quenching, and is to be precipitated homogeneously and finely during precipitation annealing. The quality that can be achieved is documented by means of examples as well as in the form of tables of mechanical properties.
According to this patent, the means of substitution mixed-crystal alloying as well as precipitation hardening is used in conjunction with the means of dispersion hardening, cited in column 1, line 65 of the description, for increasing the mechanical strength of such alloys. The strength values that are realized are thus the result of two or three strength- and hardness-increasing processes going on simultaneously.
The relatively small amounts of O, but also N and/or C in the alloy indicate that the precipitation of oxides as a means for increasing strength plays a relatively minor role in that case. In Example 1, mention is made of remelting the ingot six times in order to assure a useful--but, due to the process used, certainly not good--homogenization of the metals and dispersoids. Even so, the process is comparatively expensive. After melting and also still after hot reshaping, such alloys have a relatively coarse grain which degrades material strength. For this reason, the description in column 1, line 015 etc. expressly warns "not to prolong the solution annealing of the sheet metal unnecessarily in order to prevent grain growth".
Room temperature data are not given in the description. Experience shows that in alloys produced with this process, relatively high strength can be expected, but at the same time ductility at room temperature will be low (see e.g. V. G. Grigorovich and E. N. Sheftel, Met. Sci. and Heat Treatment 24 (7-8), p. 472 (1983).
U.S. Pat. No. 3 181 966 describes a basic niobium alloy containing 0.25-0.5% oxygen and/or 1-3% zirconium and/or titanium, with a weight ratio of oxygen/titanium or zirconium between 3:1 to 12:1. In that case, strengthening of the material is achieved by means of oxide dispersion hardening, plus, corresponding to the examples quoted, also to a certain extent by oxygen in interstitial solution and by alloying niobium with titanium and/or zirconium. It is pointed out there that higher contents of oxygen in interstitial solution will cause great brittleness in the niobium. In order to counteract this effect, that process makes use of metal oxides of metals having a higher bond energy (negative bond enthalpy) than that of the basic metal only in the presence of excess oxide metal. The additions are added, e.g. as titanium oxide powder and spongelike titanium metal, during an arc remelting process of the highly purified niobium. The process of cooling, which is important for the form of dispersion precipitation, is paid no attention in the patent description. This process does not permit any very fine distribution of the dispersoids in the basic metal.
The feasible strength properties of niobium alloys that had undergone additional hot reshaping but no recrystallization are summarized in a table and compared with the properties of pure, commercially available niobium qualities. They will later serve as comparative data for the strength increases possible with the process of this invention.
The invention at hand has as its objective the development of a process to produce ODS sintered alloys having high ductility and strength properties, using a high-melting basic metal, which is more economical than known processes. The strength properties of alloys produced with known metallurgical processes should be at least equaled, both in the deformed and in the recrystallized state, without making use of the formation of substitution mixed crystals or of the classical precipitation of a second metal or compound phase as means to achieve increased strength.